Since this is your niece I assume her parents will be packing her clothes and things, right? Are you wondering what sorts of things you'll want to have on hand while traveling or on park days?
I have few things in my bag or purse all the time for my 6 and 2 year old:
A quick snack, this is the most important thing because kids can become ravenously hungry in an instant and a hungry kid is a grouchy kid. I bring a granola bar or fruit leather or a bag of nuts, or goldfish crackers, mints, something quick and easy that I can just carry around all day and then pull out when we need it. I carry multiple snacks, you never know... Depending on the kind of bag I'm carrying I might also have a small bottle of water with me, that would be a must for Disneyworld.
A small notebook and a pen or some crayons, this is unbelievably useful to pass the time in lines or while waiting for a meal. She can draw, you can play tic tac toe, there are all kinds of possibilities.
Band-aids. Even if there's no blood, a band aid makes everyone feel better. Especially a My Little Pony band-aid, it's like magic
Tissues, napkins or wet wipes.
Sunscreen.
Fanny Pack? On our last trip our daughter had just turned 6 and we got her her own fanny pack. I ordered one with Tinkerbell on it off of
Amazon and stocked it with some of the things I normally carry like snacks and a notebook. She also got a small, inexpensive camera for her birthday so she carried that too. It was great! It might be a fun vacation surprise for your niece, then she has control of it if she gets hungry or bored.
Patience. She's still young enough to need to drift a little through the parks, go slow, sit in the shade and eat a Mickey bar, look for hidden Mickeys take breaks, splash around in the water play areas and smell the roses.
It's a lot of fun to take in Disney with a kid, especially for the first time. You'll probably see and do a lot of stuff you've never even paid attention to before. It'll be great.